In one type of thermal printer, a dye-donor element is placed over a medium for receiving an image. The superimposed elements cooperate with a printhead having one or more lasers. When a particular laser is energized, it causes dye from the donor to transfer to the medium. The density or darkness of the colored dye transferred to the medium is a function of the energy delivered from the lasers to the donor. The lasers are usually arranged in an array of diode lasers which can be selectively actuated to direct radiation onto the dye donor. The laser array forms successive swaths of scan lines on the medium as the laser array and the medium are moved relative to each other.
Because of the high resolution of such imaging systems, there is a very small depth of focus. The focus position can vary due to various characteristics of the system, for example, flatness of donor and medium, roundness of drum, etc. This creates a need for detecting the distance from the laser to the donor and changing the focus position without compromising the image quality.
There are several different methods for changing the focus. One method is moving the donor. However, this is very cumbersome. The donor and medium are attached to a large rotating drum which has significant inertia, which makes this option difficult to accomplish rapidly. Another option is moving the reduction lens, which focuses the laser on the donor. This is easier than moving the donor, however, the reduction lens is also relatively massive, and it is again difficult to move rapidly.
It is also possible to move one or more elements in the reduction lens. This is feasible, and is currently used in some systems. In one prior art apparatus, the last lens element in a nine element reduction lens is mounted on flexures and is moved relative to the drum. This type of system, however, places restrictions on the size of the last lens element. Also, moving a lens element is difficult to do without adding aberration to the image. Another disadvantage is that if an autofocusing mechanism is incorporated in such a system, it must be near the drum, which rotates at high speed and creates air currents that interfere with moving the lens element to the appropriate position.
Another alternative is moving the laser to adjust the focus, however, this also creates problems. For example, in some laser printers, laser beams are directed through fibers which are aligned in an array. The fiber faces are then imaged to the donor plane. Multi-mode fibers used in this system have diameters of 50.mu. to 100.mu.. The spot size needed for graphic arts printing is between 10.mu. and 20.mu., therefore, the optical system used must image the fibers at a magnification of between 0.5x and 0.25x. Moving the fibers to change the focus position presents several challenges. First, to change the focus position at the donor plane by a distance "X" the fibers must be moved by a distance equal to X times the square of the magnification. Thus, if the optical system reduces the array of fibers by a factor of four, the fiber array would be required to move 16 mm to change the image position by 1 mm. It is also difficult to move the fibers without breaking them or tilting them.